1. Field of the Invention
The present invention relates to an aluminum oxide (Al2O3) sintered body, and a semiconductor manufacturing apparatus member and a liquid crystal manufacturing apparatus member, each of which uses the aluminum oxide sintered body.
2. Description of Related Art
At present, aluminum oxide sintered bodies are widely used in electronic parts and industrial machinery parts. Since aluminum oxide sintered bodies used for these purposes require excellent electrical properties, mechanical properties, chemical resistance, and plasma resistance, low soda aluminum oxide whose Na2O content is not more than 0.1% is commonly used. In the manufacturing process of the low soda aluminum oxide used herein, the prevailing practice is to add, when firing aluminum hydroxide that is the precursor of aluminum oxide, silicon oxide particles and aluminum chloride in order that Na2O contained in the aluminum hydroxide is removed by adsorption or reaction.
In respect to an aluminum oxide sintered body used in semiconductor manufacturing apparatus members, Japanese Patent Application Laid-Open No. 6-157132 describes a method of manufacturing a high purity aluminum oxide sintered body whose impurity content is reduced significantly. The method described includes: suppressing the alkali metal oxide in the aluminum oxide sintered body to not more than 50 ppm and the magnesium oxide content to 100 ppm; and in addition suppressing and controlling the entry of alkali metal from the manufacturing steps. Thereby, impurities become lessened and abnormal grain growth of aluminum oxide can be suppressed to attain the aluminum oxide sintered body excellent in plasma resistance.
Additionally, Japanese Patent Application Laid-Open No. 5-217946 describes a semiconductor manufacturing apparatus member that absorbs less microwave, prevents occurrence of cracks, and has excellent productivity. That is, an aluminum oxide sintered body having aluminum oxide of not less than 99.9%, silica of less than 100 ppm, and alkali metal oxide of not more than 50 ppm can have excellent plasma resistance and prevent the occurrence of cracks due to microwave, provided that a specific gravity is not less than 3.96, a mean particle size is not less than 10 μm, and a dielectric dissipation factor is not more than 8×10−4.
Recently, in industrial machinery parts, especially semiconductor manufacturing apparatus members and liquid crystal manufacturing apparatus members, as devices become larger, parts used in these apparatuses are also increasingly larger. Aluminum oxide sintered bodies used therefor are no exceptions.
The problems especially in manufacturing a large aluminum oxide sintered body are forming and firing. When a forming body is fired to obtain a large aluminum oxide sintered body having a large wall thickness, a temperature distribution occurs at the forming body interior and the surface vicinity in the firing step, thereby facilitating advance sintering of the surface vicinity. Therefore, the surface vicinal portion and the interior often have different sintered states. Specifically, due to the fact that sintering does not proceed uniformly, the surface vicinity initially becomes a dense sintered body, while in the interior crystals fail to have sufficient grain growth, resulting in a sintered body with numerous voids. To solve this phenomenon, attempts are being made, for example, by the manufacturing method described in Japanese Patent Application Laid-Open No. 6-157132, and by extremely delaying firing temperature rise in the temperature range within which sintering proceeds. But in practice, no satisfactory results have been achieved, and therefore poor productivity.
In the aluminum oxide sintered body obtained by the manufacturing method described in Japanese Patent Application Laid-Open No. 6-157132, alkali metal entering from the manufacturing steps is suppressed and controlled. However, in normally used aluminum oxide powder, material containing phosphorous is used in soda-removing step for making aluminum oxide obtained by Bayer's process into low soda aluminum oxide, and therefore, in general, the obtained low soda aluminum oxide also contains phosphorous, and the phosphorous content is as much as over 0.0025% by weight. This hinders a uniform grain growth especially in a large aluminum oxide sintered body.
The aluminum oxide sintered body described in Japanese Patent Application Laid-Open No. 5-217946 has the following characteristics: Specific gravity is not less than 3.96; Mean particle size is not less than 10 μm; and Dielectric dissipation factor is not more than 8×10−4. However, as in Japanese Patent Application Laid-Open No. 6-157132, there is no finding with regard to grain growth inhibition due to phosphorous contained in low soda aluminum oxide serving as raw material.
Thus, in the conventional aluminum oxide sintered bodies, because the surface vicinity and the interior have different sintered states as above described, there arises a large difference in density between the surface vicinity and the interior. Therefore, the obtained aluminum oxide sintered body has a large dielectric dissipation factor value. The use of such an aluminum oxide sintered body in semiconductor manufacturing apparatus members degrades microwave permeability and hence the aluminum oxide sintered body generates heat, which can cause cracks to reduce the lifetime of the semiconductor manufacturing apparatus members.